This invention relates generally to a thermal printer drive circuit and more particularly to a thermal printer drive circuit which operates from batteries and provides a uniformly high quality of printing from a print head having a plurality of thermal pens. To effect thermal printing, the thermal pen is energized for short periods of time. The heat generating elements must be heated to a predetermined temperature for every activation in order to obtain a stable printing quality. Constant temperature conditions are provided at the thermal pen when the energy dissipated is maintained constant, that is, an equal energy or equal temperature characteristic is provided.
In a multiple pen printer, the drain on the power supply battery varies depending upon the number of pens simultaneously in operation and this number varies when the print head is used to print characters with dots in a matrix, for example, a matrix of five by seven dots. When many pens operate simultaneously, the internal resistance of the battery causes a voltage drop, and battery aging also can cause a voltage drop. When the supply voltage drops, the heating current must be applied to the thermal pen for a longer time period in order to maintain the equal energy/equal temperature characteristic. In the prior art it has only been possible to provide a thermal printer drive circuit which satisfies the equal temperature characteristic over a narrow range of voltage variations.
Beyond that narrow range, at both higher and lower voltages, excessive energy is provided to the thermal pens. This is not suitable for controlling heating generating units such as the heat generating elements of the thermal printer. These components are easily subject to damage. This disadvantage may be overcome by using a constant voltage circuit for driving the thermal printer head. However, this has a disadvantage in that when using a battery as a power source, power consumption in the constant voltage circuit is quite large in itself. Thus, this approach is not suitable for driving a thermal printer operating on batteries. Accordingly, such a thermal printer drive circuit is not well adapted to a thermal printer having a large plurality of thermal pens, many of which may be required to operate simultaneously.
What is needed is a thermal printer drive circuit suitable for use with a thermal print head having a large plurality of thermal pens, and printing with high quality resulting from a constant temperature/constant energy characteristic.